The present invention relates to dual rotary actuator drive systems, and more particularly to a load transfer arrangement for balancing loads on tandem actuators connected to a common shaft.
Electromechanical actuators, and in particular direct coupled actuators which have come into increasingly common use, have a wide variety of applications. Generally speaking, actuators receive a control signal and mechanically reposition a final control element in response to that control signal. For example, in the heating, ventilating and air conditioning (HVAC) industry, actuators are commonly used to control positioning of dampers and valves in response to a signaling device, such as a thermostat. These dampers or valves, in turn, may be used to control building fluid or air flow, roof top exhaust fans, supply fans, variable air volume equipment, internal exhaust systems, cooling towers, combustion air inlets for boilers or furnace rooms, steam, hot water or chilled water lines, to name but a few.
Regardless of the specific application, the damper or valve generally includes a control shaft, the movement of which dictates a position of the damper or valve. For example, rotation of the control shaft will cause the damper to move between an open position and a closed position. Conventional actuator systems typically employ some form of linkage arrangement between the actuator output and load control shafts. Direct coupled actuators typically have an output hub which is mated directly with the damper or valve control shaft, eliminating the need for an auxiliary linkage assembly.
Mechanical stops are normally provided to limit rotation of the drive hub to a desired range. The mechanical stops, as well as the drive motor, are contained in a housing. In a typical installation, the actuator housing is mounted to a rigid support structure associated with the damper or valve being controlled. For example, the actuator housing may be mounted to duct work extending from the damper or valve in question. The fixed actuator housing provides a reaction structure for the moment load seen by the drive hub.
Most commercial damper and/or valve applications requirements can be met by one of several "standard" actuator sizes. As a result, actuator manufacturers typically produce actuator models having a limited number of torque outputs or "ratings". For example, commonly available actuators include 25 lb-in (3N-m), 50 lb-in (6N-m), 142 lb-in (16N-m), 150 lb-in (17N-m) and 300 lb-in (34N-m)
While a 300 lb-in (34N-m) actuator is sufficient to control most commercial dampers and valves, certain applications will invariably require a greater actuator torque rating. For example, a large cooling tower may necessitate a uniquely sized damper having increased output shaft torque requirements for maneuvering between an open and closed position. Because these types of applications are relatively uncommon, actuator manufacturers cannot provide actuators having a torque rating greater than 300 lb-in (34N-m) on a cost-effective basis. As a result, for dampers and/or valves requiring an output shaft torque of greater than 300 lb-in (34N-m), two (or more) of the available actuators are coupled to the output shaft.
In theory, mounting two actuators (or "tandem mount") to a single output shaft will result in the necessary torque being applied to the output shaft. Unfortunately, however, certain complications may arise. For example, during installation of direct coupled actuators, it is virtually impossible to mount the drives of both actuators to the output shaft at precisely the same rotational position. As a result, the mechanical stops associated with each actuator are not aligned. During use, then, a first one of the actuator drives will reach its mechanical stop before the second actuator drive. At this point, the mechanical stop associated with the first actuator resists further rotational movement of the first actuator drive. The second actuator continues to drive towards its mechanical stop. Effectively then, the two actuators are driving into the first actuator's mechanical stop, resulting in dramatic actuator drive wear. A similar problem may arise as a result of certain inherent inconsistencies in the electrical and gear train characteristics of the two actuators. These internal inconsistencies may cause one actuator to initiate an output shaft torque operation before the other. In this instance, the first actuator drive will reach its mechanical end point ahead of the second actuator. The first actuator's mechanical stop will resist any further movements. Once again, the second actuator will attempt to continue rotational driving of the output shaft until its determined stop point is reached. The two actuators are effectively both driving against the mechanical stop of the first actuator resulting in actuator wear and premature failure. This problem is compounded where the output shaft itself applies a load against the mechanical stop. Finally, the internal inconsistencies (and resulting non-linear operation) may result in the tandem mounted actuators "fighting" one another throughout a torque application operation.
Commercial damper and valve control actuators, and in particular direct coupled actuators, continue to be extremely popular control devices. However, due to the generally standard sizes of actuators currently available, the torque requirements of certain applications cannot be met by a single actuator. In these cases, a tandem mounted actuator approach is normally used, leading to potential concerns. Therefore, a substantial need exists for an apparatus and method for facilitating proper operation of tandem mounted actuators.